As one of the main launching methods of vehicle-missile, the key issue in missile launching is the processing safety. The past research usually used the rigid body model in the safety analysis, and the coupling effect of the launch vehicle on the launch process was not considered, and the effect of missile gas jet on the launch box was less considered. A complete coupling model of stiffness and flexible is established. Considering the coupling effect of the vibration of the launcher on the launching process, the launcher is flexible and the dynamic analysis of the model is carried out. The influence of stiffness of vertical cylinder, different thrust loads, thermal emission gas flow of missile and missile design deviation on the safety of missile out of the box is analyzed. The analysis shows that the stiffness and thrust load of the vertical cylinder have a great influence on the exit posture, and the regular conclusion of the influence of various parameters on the exit process is obtained.